A. Field of the Invention
The present invention relates to spinal fusions and more particularly to an anterior approach implant system for fixing a stabilizing appliance to the L4-L5, L5-S1 and L4-L5-S1 vertebrae.
B. Prior Art
Over 200,000 spinal fusions are performed each year in the United States, with the L4-L5 and L5-S1 disc spaces being the most commonly fused. The indications for fusion range from traumatic instability to degenerative spinal stenosis. Procedures which require decompression in addition to fusion are most commonly performed posteriorly. Fusions performed for primarily mechanical axial back pain from degenerative disc disease are done anteriorly.
In the past, performing anterior fusion with total disc excision has been problematic without using posterior instrumentation. There was the “cage rage” of the 1990's which lead to high non-union rates and failures. Surgeons then began to perform 360s or anterior and posterior fusions because of the superiority of posterior instrumentation. This subjected patients to long procedures with significant morbidity.
Recently, surgeons have been attempting to perform discectomy and anterior fusion with posterior approaches, such as posterior lumbar interbody fusion (PLIF) and transpedicular lumbar interbody fusion (TLIF). These techniques are difficult and require significant nerve root retraction, which places the root at risk. There has also been a high rate of non-union because of inadequate disc space preparation from the posterior approach. This has led a significant number of surgeons to once again approach this problem anteriorly; however, rather than using a stand-alone technique they are using anterior instrumentation.
The anterior approach allows a more safe and speedy access to the disc and more safe preparation of the disc, resulting in higher fusion rates. Unfortunately, current instrumentation systems for anterior fusion, such as the Danek triangular plate and the Synthese plate, have serious flaws.
The Danek triangular plate is an anterior plate designed for L5-S1 fusion only, with its base having two screw holes for placement in S1 and its top having one hole for screw placement in L5. Because of this triangular structure with one hole at each apex of the triangle, coronal rotation of L5 cannot be controlled.
With the Synthese anterior plate each bone screw has distal portion threads for engagement with the L5 and S1 vertebrae respectively, and proximal portion threads for engagement with the plate. The plate, the screws and the L5 and S1 vertebrae become joined as a unit, and the vertebrae cannot angulate relative to the plate or relative to each other, thus the term “fixed angle device”. Because of this construction, the Synthese plate has two significant shortfalls. Firstly, a fixed angle device does not allow any dynamic loading in compression and may stress shield the graft, possibly leading to non-union. Secondly, the locking mechanism with the Synthese plate fixes the screw to the plate and does not allow the plate to be lagged to the bone securely. This can lead to prominence of the plate, which can be a problem in this location, since the plate is placed under the great vessels at L4-L5. Also, the surgeon has no feel for the purchase of the screws to the bone.